You probably know Brix Smith-Start as the former rock star who is now a firm fixture on the front row at London Fashion Week, usually with her her pugs Gladys and Pixie tucked under her arm. You may also be aware that Brix co-owns a fabulous designer boutique called Start in Shoreditch with her husband, Phillip. That means that, despite appearances, fashion month is not all fun and front-row games for Brix but she spends a lot of time with her womenswear buyer Lizzie travelling around the key cities to choose new stock for her store. This season, I went along to a couple of buying appointments with the Start ladies while they were in Paris to get a little insight into what the other side of fashion week is like. I see plenty of buyers doing the rounds while I'm at showrooms but you never know quite what goes on. As a journalist, I'm vying for the big picture- what the look and feel of the collection is, as well as key pieces which are usually the "wow" catwalk creations. The crossover with the buyers comes from working out what it is we will all want to wear, what will be the key pieces and what works with the trends. As Brix and Lizzie explained though, it gets more even more specific than that for them because it's about knowing how their customer will want to interpret the latest looks as well as investing in pieces which will still be beautiful and wearable 5 years later.

Brix and Lizzie in Paris

Our first stop was the mammoth Comme des Garcons showrooms in Place Vendome. Lizzie and Brix wanted to buy more of the brand's accessories and perfumes for Christmas and beyond, "they're such great price points for something which is so special and will really last, you'd love that as a gift wouldn't you?" Brix enthuses as we look at the brightly coloured purses and clutches. Errr, yes! Adrian Joffe, who is married to CDG designer Rei Kawakubo, greets Brix warmly while Lizzie goes through some paperwork with a member of the sales team. We quickly spy a shelf of CDG's famous wallets which have been updated with a spacey star print. Lizzie and Brix have seen them before but are still really excited about them. To demonstrate just how important these are, Brix removes her brogue and sock to show me her star print pedicure which she asked for the week before. "This is collective consciousness" she tells me, "it came into my head and now it is on these wallets which I'm going to sell in my shop. That's a really important thing to be turned on to, and you either have it or you don't".

CDG star prints + Brix's pedicure = Collective Consciousness

Lizzie vies for the fragrance shelves where she picks out some giant bottles of cologne which she says will be good for Start's men's christmas offering. There are also beautiful, pebble like limited edition fragrance bottles on display which Lizzie and Brix immediately agree on- "they're so special and you would keep them forever". Some of the number crunching which is a big part of buying comes into play as Lizzie works out which styles and colourways of all the things they love will realistically work in store. There are spongy, spotted gadget cases and metallic zipped leather wallets which all look great, but Start can't have them all. Lizzie explains that she decides there and then what she wants but will send through the quantities she wants to order a few days later.

Keep forever Comme des Garcons art perfume bottles

After we've finished at CDG, Brix and Lizzie explain a bit more to me about how they buy a season. "You have three stages now to each season" Lizzie tells me, "Maison Martin Margiela do a pre-pre-collection so that's where you start to get the first ideas. At the show, the ideas are very directional. There might be a white shirt with huge, voluminous arms and shoulders. It's amazing, but we need to find a white shirt with a gentle version of that". Brix adds, "it's about something with a breath of Margiela magic but not the whole shebang". Later, trends begin to emerge which Start want to buy into but "we need to work out who is doing it best for the best price" Lizzie says. They also stress the importance of perceived value, "If I'm spending £700 on a dress it must look like it cost that much" says Brix, "the absolute best thing, which makes me so so happy is when a woman spends that on a dress in my shop and then comes up to me 5 years later and is like 'I love this dress and it is still making me happy.' "

Brix's ultimate test for a product is the "fashiongasm". That is, if something fills her with an explosive, must-have sense of need then she knows she's onto a hit. FYI, Brix's recent fashiongasms have come from NEWGEN shoe designer Sophia Webster and the leather tote bags of Sophie Hulme. I think she has another when we move on to Alexander Wang's showroom. While we're perusing the shoe offering, Brix spies a pair of black ankle boots which have a cutaway heel, lined in rose gold. She already has a pair but is pleased to see them again and describes wearing them as "having angels nibbling at my heels"- I'm not sure anyone could have put it better.

Wang is more chaotic than Comme and we have to wait a while before a showroom representative comes to help. In the meantime, Lizzie and Brix are plotting their strategy for this appointment. They've already bought quite a bit of Wang's pre-collection. In fact, the way it usually works out means up to 70% of their buy comes from pre collections (and pre-pre in the case of Margiela) and the remaining 30% is made up of the main collections we see on the catwalks.

A black shorts and top combo with sliced out segments sewn together with transparent thread is pointed out to Lizzie and Brix when they ask what the big sellers have been so far but they're not so sure it will work for their customer. However, when the model comes out wearing the pieces it looks like they might change their mind.The models are crucial because they show buyers how the pieces are cut and hang, it's not fail safe but if something doesn't work on a model then it's not going to work full stop. Lizzie and Brix dismiss a few pieces because "you could never wear a bra with that"and think about all the ways to wear items they like, they go through the options for making the lemon knit tank work and what you could wear with it.

I learn that buying is about being quick, decisive, sure of your opinions but open to changing your mind and noticing the world around you. Brix tells me about the Isabel Marant Bekett wedge trainer phenomenon, "I remember going to the showroom and I actually preferred the Betty style but every girl in the showroom was wearing the Bekett, it felt like the hot thing. Nobody ordered enough that season, it was crazy". It's the perfect example of rising above your own level of taste to detect the general mood, "it was inelegant but actually really cool". Lizzie thinks a similar thing is happening at the moment with Kenzo, "I'd written it off as a very 80s brand which kind of belonged there" she tells me as they're about to head off to their buying appointment at the label, "but I'm adjusting my vision because there is so much buzz about it now". Funnily enough, minutes after I leave Brix and Lizzie I see two very cool Parisiennes with two giant Kenzo bags apiece slung over their shoulders.

Thank you to Brix and Lizzie from Start. Visit the store at 42-44 Rivington Street, London, EC2 3QA.